Figure 5.

CaBP1 prolongs Ca²⁺ currents through Ca_v1.2 channels and does not support Ca²⁺-dependent inactivation. A, Effect of CaBP1 on Ca_v1.2 Ca²⁺ currents evoked by step depolarizations. Traces represent Ca²⁺ currents evoked by a 1 sec step from -80 to +10 mV in whole-cell patch-clamp recordings of HEK293T cells transfected with Ca_v1.2 subunits with or without CaBP1. I_res /I_pk was determined by dividing the residual current amplitude at the end of the pulse by the peak current amplitude. Extracellular solution contained 10 mm Ca²⁺, and intracellular solution contained 5 mm EGTA. Data represent mean ± SEM for Ca_v1.2 (n = 17) and Ca_v1.2 plus CaBP1 (n = 11) (^*p < 0.001). B, Impact of CaBP1 on fast and slow inactivation. Ca²⁺ currents obtained in A were fit with a double-exponential function. The fast and slow time constants (τ_fast,τ_slow) and fraction of channels showing fast and slow components of inactivation (F_fast, F_slow) were averaged and shown for cells transfected with Ca_v1.2 alone or Ca_v1.2 plus CaBP1 (^*p < 0.001). C, Comparison of Ca²⁺-dependent inactivation in cells transfected with Ca_v1.2 alone or cotransfected with CaBP1. Traces show currents evoked by 1 sec depolarizing step from -80 to +10 mV for I_Ca (black trace) or 0 mV for I_Ba (gray trace) to compensate for -10 mV voltage shift for I_Ba. Extracellular solution contained either 10 mm Ca²⁺ or Ba²⁺.[I_Ca - I_Ba] represents the difference between I_res /I_pk for I_Ca and the mean I_res /I_pk for I_Ba for Ca_v1.2 channels alone (n = 17; open bar) or Ca_v1.2 plus CaBP1 (n = 11; hatched bar) (^*p < 0.001). D, U-Shaped voltage dependence of I_res /I_pk for I_Ca in cells transfected with Ca_v1.2 alone but not in cells cotransfected with CaBP1. I_res /I_pk was determined for I_Ca evoked by 1 sec test pulses from -80 mV to various voltages, averaged, and plotted against test voltage for Ca_v1.2 alone (open circles; n = 9) or Ca_v1.2 plus CaBP1 (filled circles; n = 8).